Electric cars
The induction motor is very effective and used in both EVs and hybrid cars, but an alternative can be used to provide a better starting torque: the permanent magnet motor (PMM). This is similar to the induction motor but involves replacing the bars of the rotor with permanent magnets which creates a combined magnetic force to interact with the RMF. In order to generate maximum torque, the combined magnetic force is placed at 45 ° to the RMF. With the forces of the opposite poles attracting and the like poles repelling almost tangential to the rotor and in the same direction you have an optimal torque; this is controlled by a smart controller. This is suitable for starting and climbing hills but doesn’t fare so well at high speeds due to the back EMF. Back EMF occurs when the magnetic field of the permanent magnets interacts with the copper windings generating a reverse voltage to the stator supply voltage. This is directly proportional to the rotor speed and can result in eddy current loses which can increase temperature in the motor. 7 The third motor which can be used is a synchronous reluctance motor (SynRM). Reluctance: ‘a medium’s ability to oppose magnetic fields’ is used in this motor. Iron has a low reluctance, but air has a high reluctance, and so when slots are cut in the iron rotor, it will spin with the RMF as the rotor is eager to maintain a low reluctance state, with the slots of air in the rotor not interfering with the magnetic field. SynRM motors are suitable for high speeds and do not experience back EMF. 8 In order for the EV to perform optimally at all speeds Tesla recently combined the PMM and the SynRM by inserting 4 small permanent magnets deep into the slots in the rotor reducing the back EMF. This is called the internal permanent magnet synchronous reluctance motor (IPMSynRM) and is used in the Tesla Model 3. Tesla determined to that the RMF should be at 50° to obtain maximum torque and chose a six-pole magnetic field to increase this further. Additionally having 4 small magnets in each slot oppose to one solid motor reduces demagnetization and heat. 9 All of these motors, unlike ICEs, use electricity to run and are powered by thousands of small Lithium- ion cells. These cells are made of four main components: the anode, the separator, the cathode and the electrolyte. The anode is made of a lithium cobalt oxide (LiCoO 2 ) attached to an aluminium sheet and the cathode is made of graphite attached to a copper sheet. In between this is the separator, a thin layer of micro-perforated plastic, 10 and the electrolyte is often a non-aqueous solution, most commonly Lithium hexafluorophosphate (LiPF 6 ) dissolved in organic carbonates. 11 These three sheets are tightly coiled into a small, nickel-coated steel casing 9 with the electrolyte liquid. 12 When the cell is charging, the positive lithium ions are attracted to the cathode and travel through the electrolyte, as the electrolyte does not allow the electrons to pass through, they are attracted to the anode and will flow through the external circuit to the cathode. When all the lithium ions and electrons have collected in the graphite sheet by the cathode, the cell is fully charged. When the cell has stopped charging and the power wants to be used for the induction motor, the lithium ions will move back through the electrolyte to the anode and the electrons will travel through the induction motor back to the anode. 13 A Tesla cell holds between 3.0V and 4.2V and roughly 30A, and in order to increase both the voltage and amperage,
7 Lesics 2020. 8 Ibid. 9 Ibid. 10 Donut Media 2018. 11 Qi Li 2016. 12 Lithium-Ion battery 2017. 13 Lesics 2019.
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